Different universities have different rules. Some let you start whenever, others only let you start when they tell you. In reality, most are probably flexible on start times. I expect those dates are saying that in order to start by the 1st of October, you must apply by the 1st of August. However, you could still start later than the 1st of October if you wanted to.
It's better to start at the start of the academic year though, because that's when most people start and it's when all the inductions and training are.